Method of and apparatus for generating a fine dispersion of particles

ABSTRACT

In an apparatus for generating a fine dispersion of particles in a gas, a body (47) for containing the gas has an inlet (59a, 59b) and an outlet (60) for passage of the particles. Nozzles (55) are provided to direct a plurality of jets of gas through the body (47). The body (47) is maintained at an elevated temperature by a heating element (49).

This application is a continuation of application Ser. No. 07/408,245,filed Sep. 18, 1989 (now abandoned).

The present invention relates to a method of and an apparatus forgenerating a fine dispersion of particles in a gas, the apparatuscommonly being termed `a smoke generator`. In particular, the presentinvention relates to a method of generating a fine dispersion of glasspowder to be electrostatically deposited on the internal surface of aglass envelope for a lamp. The method and apparatus of the presentinvention are particularly suited for use with the method and apparatusfor producing a light diffusing coating in a glass envelope as disclosedand claimed in our two copending European Patent Applications claimingpriority respectively from GB Patent Application Nos. 8821771.6 and8821800.3 and from GB Patent Application Nos. 8821777.3 and 8821800.3.

GB 1395437 (Philips) discloses an apparatus for electrostaticallydepositing powder on the inside surface of the envelope of a lamp whichthe powder to be deposited is mixed with a transport gas stream. In oneembodiment, a container holding powder includes a mechanical stirrerpositioned within the bulk of the powder by means of which a cloud ofpowder can be formed above the powder level.

Proposals to use a fluidised bed to create a fine suspension of dustpowder an to turn that suspension into an aerosol are not new, GB2130906A (NRDC) discloses an apparatus for dispensing dust suspensionsto simulate industrial fume emissions such as occur in the iron andsteelmaking industry. The apparatus includes a cylindrical vessel whichis spanned by a perforated baffle. A bed of particles includingdeagglomerated dust particles and nickel spheres is formed upon thebaffle. Dry and filtered air enters the vessel from below the baffle andpasses through the baffle and bed of particles. This passage of airthrough the bed forms a rising suspension in air of fine particleselutriated (ie separated from the heavier particles) from the bed.

GB 1421517 (Interliz) discloses an apparatus for `whirling-up` a powderyor granular material, such as magnesium oxide, in a vessel and forblowing it out of this vessel. One nozzle mounted in the upper end ofthe vessel is directed downwardly and substantially in thecircumferential direction of the vessel. A further nozzle at the lowerend is directed tangentially. Passage of air through the lower nozzleand into the powdery material causes the material to be loosened up,particularly at the surface, in the course of which a cloud of powderforms above the surface. This cloud is `whirled-up` by air from theupper nozzle. Hot air at about 100° C. may be used to effect at least apartial drying of the material during the process, which is particularlybeneficial for the hydroscopic magnesium oxide.

It is an object of the present invention to provide an improved methodof and apparatus for generating a fine dispersion of particles in a gas.

According to a first aspect of the present invention there is provided amethod of generating a fine dispersion of particles in a gas comprisingthe steps of:

maintaining a body containing the gas and the particles at an elevatedtemperature; and directing a plurality of jets of the gas through theparticles.

The inventors surprisingly found that when the temperature of theirapparatus for generating a fine dispersion of particles was increasedfrom room temperature to 220° C., there was an approximately ten-fold inthe mass of particles taken into suspension by a given mass of gas. Thisincrease was much greater than could be explained by considering thethermal expansion of a constant mass of gas with increase intemperature. A similar increase of approximately tenfold was found in asubsequent test when the temperature of the apparatus was increased fromroom temperature to 100° C. The maximum temperature which could be usedwould be dependent on the properties of the powder to be put intosuspension.

The powder used was a zinc borosilicate glass composition containing byweight 22% silicon dioxide (SiO₂), 31% boron oxide (B₂ O₃), 27% zincoxide (ZnO), 4% sodium oxide (Na₂ O), 7% sodium fluoride (NaF), 5%potassium oxide (K₂ O), 2% molybdenum oxide (MoO₃) and 2% tungsten oxide(WO₃) with the addition of 0.3% by weight of the free-flow aid DegussaD17. The powder was very finely divided with an average particle size offrom 3 μm to 4 μm and a maximum particle size of 12 μm.

In a preferred embodiment, the plurality of jets are directed toconverge at one or more points in the body containing the particles.This increase in efficiency of the method because of the turbulenceproduced in the bulk powder by the convergence of jets and also becauseit reduces the risk of a jet gas blowing straight through the bulkpowder.

According to a second aspect of the present invention there is providedan apparatus for generating a fine dispersion of particles in a gas, theapparatus comprising:

a body for containing the gas, the body having an inlet and an outletfor passage of the particles;

means for directing a plurality of jets of gas through the body;

and means for maintaining the body at an elevated temperature.

BRIEF DESCRIPTION OF THE DRAWING

An embodiment of the present invention will now be described, by way ofexample only, and with reference to the accompanying FIGURE which showsa sectional view through the embodiment.

A smoke generator assembly 44 has a main body 47 for containing glasspowder 48 and gas. A metal clad heating element 49 surrounds the body47, maintaining the body 47 and its contents at an elevated temperaturethe whole being surrounded by heat insulation 50. Springs 51a, 59b areprovided to enable the assembly to be vibrated during operation.

Pressurized air is fed into the smoke generator via an inlet 52 and aspiral groove 53 that runs around the outside of the main body 47.Passage of air through the groove 53 allows the air to be preheated toan elevated temperature prior to its injection into the main body 47 viaa manifold 54 and nozzles 55. The FIGURE shows three nozzles 55 directedtowards a point 56. The number of nozzles 55, their arrangement and sothe number of points 56 provided in a generator assembly depends on theconfiguration of the generator.

Fresh spray powder is metered into the top of the generator at an inlet57. The power passes down ia channel 58 outside the body 47 due tovibration of the assembly and is heated by conduction of heat throughmelt parts of the spiral groove 53. The powder is drawn into the body 47via inlets 59a, 59b due to flow patterns caused by air from the nozzles55.

The point 56 to which the nozzles 55 are directed is termed a `boil`point and is positioned about two thirds the way up towards the bulkpowder level (indicated by a dotted line 48a). In operation, convergenceof jets of air from the nozzles 55 at the `boil` point 56 produces afine dispersion of powder particles in air, termed a `smoke` which movestowards an outlet 60 as indicated by the arrows D. A plurality ofbaffles 61 regulates the passage of the `smoke` out of the body 47,improving the uniformity of the density of the `smoke` generated bypreventing localised high concentrations of powder such as may arisefrom the `boil` from exiting the body 50. The baffles 61 also preventany agglomerates or large extraneous particles from exiting the body 47.

As already indicated, the design of the generator is directed towardsmaintaining the body 47, powder 48 and gas at an elevated temperature.To this effect, the gas and powder are preferably pre-heated prior totheir introduction into the body 47. It has been found that an elevatedtemperature of 220° C. has a surprisingly beneficial effect on the smokewhich can be produced. Another consideration to be taken into account isthe use to which the smoke will be put.

The air pressure maintained in the manifold 54 depend son the density ofsmoke desired, the temperature of the generator and the number and sizeof the nozzles 55. It is anticipated that in the majority ofapplications, the air pressure used would be less than one atmosphere(in excess of ambient pressure).

The arrangement of nozzles 55 within the body 47 is affected by thenumber of outlets 60 required. A generator with a single outlet wouldtypically be of a cylindrical design and have seven nozzles mounted inthe base, six arranged hexagonally about a central nozzle. A generatorwith multiple outlets would typically be of a rectangular design withthe number of nozzles determined by its length.

A variety of modifications are envisaged. Larger generator assembliesmay be provided with some form of mechanical stirring. If the powderused is not very fluid, a metered dose of heavier particles (preferablyof spherical shape) may be placed in the body 47 to assist inmaintaining the homogenity of the bulk powder. Furthermore, one or bothof the gas and powder may be preheated independently of the smokegenerator prior to their introducing into the smoke generator assembly.It is envisaged that control of both the concentration of powder in thegas and the total flow rate of the suspension can be varied widely andindependently of each other by varying parameters such as temperature,pressure, nozzle diameter, depth of bulk powder and rate of feed ofpowder into the body.

It is envisaged that the present invention may have wider applications,e.g. generating suspensions of enamel powder for dry enamelling ofmetals or external enamelling of glass envelopes or in the plasticscoating of metal components.

We claim:
 1. Method for improving uniformity in the amount of solidparticles entrained in a gas discharged from a top surface of afluidized bulk of particles in a smoke generator, comprising thesteps:introducing gas through a plurality of nozzles at the bottom ofthe bulk of particles to provide upwardly directed gas jets within thebulk to maintain the bulk of particles in a homogeneous, turbulentfluidized state; discharging the gas entraining such solid particlesfrom the top surface of the fluidized bulk of particles; directing thegas jet to intersect each other within the fluidized bulk at one or morelocations sufficiently below the top surface of the bulk to prevent thejets form streaming through the bulk for improving uniformity in theamount of solid particles entrained in the gas discharged from the topof the bulk; and maintaining the fluidized bulk of particles at anelevated temperature for increasing the amount of such dry particlesentrained in the gas discharged from the top surface of the bulk.
 2. Amethod according to claim 1 wherein gas supplied to the body ispreheated to said elevated temperature.
 3. A method according to claim 1wherein the gas is air.
 4. A method according to claim 1 wherein thebody is vibrated at a low frequency.
 5. A method according to claim 1wherein the passage of particles out of the body is regulated.
 6. Themethod of claim 1, in which all the jets are directed toward one or morepoints approximately two-thirds the way up from the bottom toward thetop surface of the fluidized bulk of particles.
 7. Method for increasingthe amount of fine dry particles entrained in a gas discharged from atop surface of a fluidized bulk of particles in a smoke generator,comprising the steps:metering into the bulk fine dry particles of amaterial for increasing entrainment of the dry particles in the gasdischarged from the top surface of the fluidized bulk when temperatureof the smoke generator is increased from room temperature to an elevatedoperating temperature substantially above room temperature; directinggas from a bottom of the bulk, upward through the bulk to maintain thebulk particles in a homogeneous, turbulent fluidized state; dischargingthe gas entraining such dry particles from the top surface of the bulk;and directing the gas jet to intersect each other within the fluidizedbulk at one or more location sufficiently below the top surface of thebulk to prevent the jets from streaming through the bulk for improvinguniformity in the amount of solid particles entrained in the gasdischarged from the top of the bulk; maintaining the fluidized bulk ofparticles at the elevated temperature for increasing the amount of suchdry particles entrained in the gas discharged from the top surface ofthe bulk.
 8. The method of claim 7, in which the elevated temperature isabove 100° C.
 9. The method of claim 7, in which the elevatedtemperature is approximately 220° C.
 10. The method of claim 7, in whichthe elevated temperature selection is dependent on the properties of thedry particle material.
 11. The method in claim 7, in which the dryparticles are of zinc borosilicate glass composition.
 12. Apparatus forgenerating a dispersion of fine, solid particles entrained in a gas,comprising:body means for containing a fluidized bulk of particles;metering means for introducing the fine, solid particles into the bulkof particles within such body means; distribution means for introducinggas into the bottom of the bulk, for providing upward directed gas inthe bulk to maintain the bulk of particles in a turbulent, homogeneous,fluidized state and for entraining a uniform amount of the fine, solidparticles dispersed in the gas discharged from a top surface of thefluidized bulk, and which includes, nozzle means for directing jets ofgas through the bulk toward the top surface and for directing such jetsto intersect each other within the fluidized bulk at one or morelocations sufficiently below the top surface of the bulk for preventingthe jets from streaming through the bulk for increasing the amount andimproving uniformity in the amount of said fine, solid particlesentrained in the discharged gas; path means for conducting the gas,discharged from the top surface of the fluidized bulk of particles andin which the fine, solid particles are entrained, out of such bodymeans; and means for maintaining the fluidized bulk of particles at anelevated temperature for increasing the amount of such dry particlesentrained in the gas discharged from the top surface of the bulk.
 13. Anapparatus according to claim 12 further comprising means for vibratingthe body at a low frequency.
 14. An apparatus according to claim 12further comprising means for regulating passage of particles out of thebody.
 15. The apparatus of claim 12, further including,baffle meanspositioned above the top surface of the bulk of particles within suchbody means and communicating with such path means for improvinguniformity in size and amount of the fine, solid particles entrained inthe gas conducted through such path means.